This invention relates to an improvement in the method for making mirrors. More specifically this invention relates to an improved method for making mirrors which produces mirrors that are less susceptible to degradation due to weathering.
Increasing costs of energy furnished by fossil and nuclear fuels, in addition to forecasted shortages of fossil fuels, have stimulated a search for ways in which modern technology can help to provide options in the use of alternative sources of energy. One obvious source of thermal energy is that derived from the sun.
A number of devices have been built or are being being designed which utilize thermal energy from the sun in some way or another. One system currently under construction utilizes a large array of individually controlled tracking mirrors to redirect incident solar radiation to a central receiver or boiler at the top of a tower where the solar energy is absorbed and converted to heat by a black body placed in the focal region. These tracking mirrors are known as heliostats and may, in a large commercial installation, occupy as much as a square mile or more of area. This is necessary in order to direct sufficient solar radiation to the central receiver or power tower to provide enough thermal energy to ultimately operate a conventional steam turbine generating station. It is vitally important to the overall efficiency of the system that the mirrors reflect as much of the incident solar energy as possible and that this efficiency be maintained for long periods of time without appreciable deterioration.
Mirrors presently under consideration include silvered float process glass and metallized thin films with a protective coating. Mirrors of silvered glass are well suited for use in solar energy heliostats since they have a higher weighted solar reflectance than do metallized thin films. They are however, after a period of exposure to the weather subject to delamination of the silver backing from the surface of the glass. This results in a substantial reduction in the amount of incident light redirected to the collector.
The silvered glass mirrors used for heliostats, are second surface mirrors which have a conventional four layer structure which has changed very little in many years. The glass serves as the substrate for the mirror deposition process and also provides a hard, cleanable surface for the finished product. Mirrors are commonly coated on 1/8 or 1/4 inch glass. A thin silver coating serves as the reflective layer and provides a flat reflectance across the visible spectrum. In conventional mirrors the silver layer is about 700 A in thickness which corresponds to about 70 mg/ft.sup.2. Over the the silver is a layer of copper, whose function is not well understood. One possible function is to serve as a stress relief layer between the silver and the outer paint layer to accomodate differential dimension changes due to paint shrinkage upon curing and thermal expansion that could occur during paint drying or normal use. An alternate function may be to serve as a sacrificial layer for preservation of the silver layer. The copper layer may also provide improved adhesion between the metal and paint layers. Generally the copper layer is about 300 A thick corresponding to about 25 mg/ft.sup.2.
An outer paint layer provides a protective coating over the metallic films to prevent damage. This includes abrasion resistance for the mirror between production and final mounting. Generally the paint coatings are about 0.001 inch thick corresponding to about 6-10 g/ft.sup.2.
The first step in the production of mirrors is to scrub the glass with an abrasive, generally a slurry of cerium oxide, in order to remove contaminents and provide a clean surface for mirroring. After rinsing the slurry from the glass, a sensitizing solution is applied to the surface. This serves to hasten the silver deposition rate and improve adhesion of the silver to the glass. The use use of stannous chloride is most common although palladium chloride is also occasionally used in place of the tin. The role of the sensitizer is not completely understood, but it is believed to form tin sites on the surface of the glass which serve as nucleation centers for the silver layer deposition process.
After the sensitizing solution has been thoroughly rinsed from the glass with deionized water and, while the surface is still wet, the silvering chemicals are sprayed on the sensitized glass. The most commonly used chemical deposition system consists of three separate solutions, a silver solution such as silver nitrate, a caustic such as sodium hydroxide and a reducer such as formaldehyde or dextrose. The chemical reaction results in the precipitation of a silver layer when the three solutions are mixed, by the simultaneous spray application of the solutions on the surface of the glass. After completion of silvering, the solutions are thoroughly rinsed from the glass to terminate the precipitation reaction and to prevent entry of residual silver solutions into the copper deposition region, which would otherwise degrade the mirror quality.
The copper layer is generally applied by chemical deposition and commonly uses a slurry of iron filings in water together with a solution of soluble copper such as copper sulfate. The precipitation reaction begins when the solutions are mixed on the surface of the silver. Other systems may also be used which do not employ the iron filing slurry. Alternatively, the copper layer may be applied by electrolyte deposition. Upon completion of the copper deposition step, the surface is carefully washed to remove the copper solutions from the surface and air dried.
The mirror is heated, usually with infrared radiation from the uncoated glass side, to partially cure the metal layers by driving residual water from them before the final protective coating of paint applied to the back of the mirror by rolling, spraying or by curtain coating. After drying, the mirror is ready for shipment.
While mirrors have been produced by the above described method for many years, they have not been completely satisfactory. For example, when the mirrors are subjected to a high moisture environment such as the out-of-doors, for any extended period of time, they are prone to debonding or delamination of the silver reflective coating from the glass surface. This causes a substantial reduction in the reflectivity of the mirror rendering them unsuitable for use as heliostats.